1. Field of the Invention
The present invention relates to a cordless telephone system operated on a multi-channel access basis.
2. Description of the Related Art
A cordless telephone system comprises a fixed station and a mobile station connected via a radio frequency (RF) link. A handset unit is provided at the mobile station, and a base unit is provided at the fixed station. The base unit contains a controller able to connect with an external telephone network. These base and handset units are connected through an RF link established by the respective antennas thereof. Thus, the handset unit can receive or originate a call anywhere in a service area of, for example, several hundred meters in diameter, and that call is sent to or received from another subscriber through the external telephone network and the base unit.
If a plurality of similar cordless telephone sets are used within short distance from each other, radio interference is apt to occur. From this viewpoint, a conventional method for establishing an RF link between the base and handset units, i.e., a single channel access, is not practical. Accordingly, a multi-channel access method for these cordless telephones has become widely used. In the multi-channel access system, before the establishment of the RF link, first a search is made for an idle channel among a plurality of previously allotted channels, and then a single idle channel is selected as the common RF link. A speech path is then established between the base and handset units via the thus selected common RF link. The present invention relates to a cordless telephone set operated under such a multi-channel access system.
In the prior art, several methods for utilizing a multi-channel access have been proposed. According to one method, both the base unit and the handset unit contain respective read-only memories (ROM's), and each of the ROM's stores the same data, which specifies the sequential order of searching the channels. Thus, the base unit and the handset unit both search for the idle channel, through access to their own ROM's, to find a common channel.
According to another prior art method, one of the channels is allotted, in advance, as a control channel, and thus the base unit can inform the handset unit of the number of the idle channel to be used for communication therebetween, by using the control channel.
According to a further prior art method, a cordless telephone set is provided with a first memory, in either the base unit or the handset unit, and a second memory is provided in the other unit. The first memory stores previously determined data specifying the group of channels to be used among a plurality of previously allotted channels. The data in the first memory is copied and transferred to the second memory during a stand-by mode, i.e., both the base unit and the handset unit are in the idle state, and thus both units can find an idle channel to be used by referring to the first and second memories. This prior art is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 60-62747 (laid-open on Apr. 10, 1985).
The above mentioned methods all have disadvantages. According to the first method, the base unit and the handset unit scan the idle channels independently and, every time a busy channel is found, perform an investigation of whether the received identification (ID) code coincides with the previously determined code. In short, the base unit and the handset unit realize their respective channel searches for catching a common channel independently of each other, and as a result, a very long time is needed to establish an RF link between the base and handset units. For example, in the worst case, it has been known to take about 10 seconds to establish the RF link if a large number of channels are used for the multi-channel access in the cordless telephone system.
The cordless telephone system of the second method is not advantageous from an economical view point, since one of the channels, i.e., the control channel, can not be used for normal communication purposes. Further, in the second method it often takes a very long time to establish an RF link between the base and handset units when a conflict arises, regarding use of the common channel, between two or more base and handset units.
According to the third method, the cordless telephone set can use only the channels previously specified by the aforesaid first memory. This means that it will take a very long time to find an open channel from among the limited number of channels available, especially under heavy communication traffic conditions when a plurality of cordless telephones sets are used in one system.
As can be understood from the above, each of the prior art cordless telephone systems using a multi-channel access has a common disadvantage of a long wait before completing the RF link between the base and handset units. This causes a related disadvantage of frequent call conflicts, and such call conflicts cause inconvenience to the user of the telephone. For example, a bell or buzzer, etc., in a base unit is driven when the base unit receives an incoming call from the external telephone network. The operator then lifts the handset unit and pushes the hook button to receive the incoming call at the handset unit. In this case, if the hook button is pushed while the bell, etc., at the handset unit is still ringing, the incoming call is received by the user in the usual fashion. If, however, the hook button is pushed before the bell, etc., rings at the handset unit, the incoming call is not received and a call conflict occurs. This is because, if the hook button is pushed before the bell, etc., rings, then the system treats this as a call origination demand by the user, and thus both an internal origination call state and an external incoming call state occur simultaneously. If the related condition continues for a predetermined time, such as 3 seconds, the handset unit detects that a call conflict has occurred, and thus the call origination demand by the user is automatically cancelled and the incoming call is retrieved. An impatient user will be irritated by this procedure for a cancellation of such a call conflict.